1. Field
The present disclosure relates to a switching device using a chalcogenide material and its manufacturing method, and more particularly, to a chalcogenide switching device using germanium (Ge) and selenium (Se) and its manufacturing method.
2. Description of the Related Art
At the present, a chalcogenide material is widely used for a phase-change memory cell as a VI group element such as tellurium (Te) and selenium (Se). The chalcogenide material exhibits a phase change between a resistivity state and a conductivity state.
Such a resistivity-conductivity change is caused by a phase change of a crystalline state and an amorphous state of the chalcogenide material. Since the crystalline state has a low resistance structure, the chalcogenide material exhibits conductivity, and since the amorphous state has a high resistance structure, the chalcogenide material exhibits resistivity. Generally, the phase transformation from the crystalline state to the amorphous state is called a reset, and the phase transformation from the amorphous state to the crystalline state is called a set. In detail, the change from the crystalline state to the amorphous state occurs according to temperature, and such a temperature change is generated by thermal energy (for example, Joule effect at current/voltage and Joule effect at the chalcogenide material) formed by an electric current flowing through the chalcogenide material and a resistive electrode which is in contact with or adjacent to the chalcogenide material. Therefore, assuming that a pulse voltage is applied to a device composed of the chalcogenide material, the phase of the chalcogenide material may be adjusted based on the pulse size.
Different from above, several amorphous chalcogenide materials show another kind of switching phenomenon, so called Ovonic threshold switching. It was firstly reported by S. R. Ovshinsky in 1968. [ref. S. R. Ovshinsky, Phys. Rev. Lett. 21, 1450 (1968)]. This switching is characterized by a huge change in the electrical resistance when the applied voltage exceeds a certain threshold voltage (Vth). Since this switching is volatile, i.e. the chalcogenide material returns to highly resistive state when the bias is removed, a switching device based on this phenomenon is being developed.
The OTS has a high resistance with respect to a voltage lower than a threshold voltage (Vth). If the applied voltage exceeds the threshold voltage (Vth), the OTS experiences a transition from the insulating state to the conducting state, and consequently, the OTS has a low-resistive state. If the voltage across the OTS drops below a holding voltage (VH) or the current flowing through the OTS drops below a holding current (IH), the OTS returns to a high-impedance condition. This operation may be symmetrical and may also be generated with respect to negative voltage and current.
However, the chalcogenide material layer used in the OTS is generally As2Se3 which uses Arsenic (As). Since As is not an environment-friendly element, it is demanded to use a new material to replace As.
In addition, in the chalcogenide switching device (namely, the OTS device), a high threshold voltage may increase an amount of voltage applied, which may increase power consumption and give a bad influence to the device. Therefore, a necessity for lowering and adjusting the threshold voltage for switching arises.